It is well known that a combined hydraulic motor and speed reducer conventionally includes a motor case, a cylindrical rotatable member rotatably supported on the outer periphery of the motor case, and a speed reducer arranged to provide a connection between a rotary shaft of the hydraulic motor and the cylindrical rotatable member. A cover is attached to an open end of the cylindrical rotatable member to form a closed speed reducer case. Lubricating oil is supplied to the speed reducer case to lubricate the speed reducer in an oil bath.
In such a conventional combined hydraulic motor and speed reducer, the hydraulic motor is fixed in position, and the cylindrical rotatable member is connected to a drive sprocket. The cylindrical rotatable member is rotated to drive the drive sprocket and lubricate the speed reducer in an oil bath. By this arrangement, the lubricating oil is stirred within the speed reducer case while the motor is being rotated. This may result in an increase in the temperature or the overheat of the lubricating oil and thus, in a decrease in the service life of seal means such an O-ring, an oil seal and a floating seal and bearings. Therefore, the hydraulic motor can not be rotated in an continuous manner.